1. Field Of The Invention
This invention relates to the in-line electron gun assembly of a plural beam color cathode ray tube, and more particularly to the focusing electrode structure thereof wherein magnetic beam shielding means are utilized.
2. Prior Art
Present day color cathode ray tubes (CRTs) commonly utilized in television and allied display applications often employ plural beam in-line electron gun assemblies wherein three separate electron beams emanate in a substantially common horizontal plane. In keeping with the present state of the art, the separate guns are compacted into a unitized assembly, from which the beams are designed to converge at the plane of a multi-opening shadow mask, and pass through the mask openings to impinge upon and excite the discrete red, green and blue color emitting elements of a phosphor array disposed on the interior surface of the tube viewing panel.
In the plural beam in-line gun assembly, the electrons emitted from separate cathodes are formed into beams, focused, accelerated and directed toward the viewing screen by a sequential arrangement of related electrodes. The design of the various electrode members of the unitized gun structure has evolved over the years into a highly sophisticated art. The size, shape, relative spacing, and materials used in the fabrication of these electrodes are influenced by a variety of considerations, the most important of which is the achievement of desired red, green and blue registration in the screen.
During tube operation, scanning of the three beams across the screen to form the red, green and blue definitive raster patterns is achieved with a deflection yoke externally positioned to encompass the neck and funnel portions of the tube envelope in the vicinity of the forward portion of the gun assembly, in conjunction with associated control circuitry. Such a combination produces varying magnetic fields within the tube which effect sequential horizontal sweeping of the beams exiting the gun to form the respective red, blue and green raster patterns. Technological advancement has produced simplified dynamic convergence circuitry for use with a self-converging yoke embodying pin cushion correction. This yoke, developed for use on in-line tubes, conventionally employs saddle horizontal deflection windings and toroidal vertical deflection windings, and as such, effects a reduction in yoke weight and material utilized. But, raster sizes are sometimes adversely affected by the introduction of aberations into the system. To the extent possible, corrections for such raster size deviations are attempted in the design parameters of the electron gun components.
There is an ever increasing trend for the manufacturer of display equipment to demand higher performance without incurring an associated increased cost penalty for cathode ray tubes. Such demands have led to increasingly sophisticated electron gun designs along with tighter tolerances on all parts of the CRT including the respective positions of the electron gun assembly and the associated external deflection yoke. Because of such high performance standards and the difficulty encountered in achieving them, for example, raster convergence, various tolerances in the tube which previously were considered acceptable have now become unacceptable. For example, in achieving raster convergence, it has been discovered that the relative positions of the in-line electron gun assembly in the neck of the tube and the externally oriented deflection yoke are extremely critical in achieving the required raster convergence demanded by the customer.
While modern unitized in-line color guns may contain as many as six or more electrodes, a commonly utilized type is the bi-potential gun wherein beam focusing is determined by the ratio of the focus electrode voltage to the respective accelerating electrode or anode voltage. A bi-potential in-line gun assembly of this type conventionally consists of a first plural-apertured planar G1 electrode positioned forward of three separate in-line cathodes; a second plural-apertured planar G2 electrode spaced forward of the G1 electrode; a third box-like electrode G3, commonly referred to as the focusing electrode, positioned forward of the G2 electrode; and a fourth or final cup-shaped G4 electrode which is often referred to as the accelerating electrode. Attached to the top of the G4 electrode is a convergence cup often containing soft magnetic materials known as shunts and/or enhancers for beneficially modifying the deflection fields of the yoke on the two outer electron beams.
The focusing G3 electrode is often formed by positioning two cup-shaped members in abutting relationship with the open tops there of facing each other to form an enclosure. Three in-line apertures are formed in the bottom of each cup-shaped member to permit passage of the respective electron beams therethrough. The longitudinal spacing between these two apertured surfaces is an important factor in the design of the electron gun structure. Various means have been used to achieve the desired spacing. For example, G3 electrodes have been formed by placing two such apertured enclosures in abutting relationship to maintain alignment of the apertures. In addition, apertured spacers have been employed between or within such enclosures in order to further adjust the overall length of the G3 electrode. While an integrated structure of this type may evidence a separate aperture plane intermediate the forward and rear aperture planes, such intermediate plane has little or no focusing effect upon the electron beams passing therethrough.
Slight adjustments of the yokes on the necks of in-line tubes have been found to be very critical for achieving desired resolution and convergence, especially in the 6 and 12 o'clock regions, of the raster.
Sometimes magnetic material has been employed in the rear portion of the G3 electrode to provide a degree of beam shielding from the toroidal yoke, which sometimes forms backfields extending into at least the G3-G4 vicinity of the gun assembly.
Accordingly, there is felt to be a continuing need to improve electron gun structures not only to satisfy increasing demands for improved performance of the cathode ray tube, but also to relieve tolerance limitations on other aspects of tube design and manufacture and, therefore, to achieve such improved performance at little or no cost penalty.